This document describes a proposal to study the epidemiology of histiocytic lymphomas, and a possibly related disease entity, immunoblastic lymphadenopathy (IBL), in Los Angeles County. The study is designed to test for multiple antecedent risk factors for these diseases, with special emphasis on therapeutic agents previously implicated. The study is a two-part case-control study. The case group will consist of 100 cases of histiocytic lymphoma identified by the Los Angeles County/USC Cancer Surveillance Program, plus 50 cases of immunoblastic lymphadenopathy identified by the pathologic consulting service of Dr. Robert Lukes. Data will be obtained by interviews conducted by a trained nurse epidemiologist. Questionnaire information will focus on therapeutic history, environmental exposures at work and at home, allergic and other possibly relevant past disease history, and family history of tumors. Medication history will be validated by contacting personal dentists and physicians of cases and controls. In addition, venous blood specimens will be obtained on a group of IBL and histiocytic lymphoma cases and controls in order to help characterize the immune responses and assist in determining the affected cell type of these patients.